Just use the offscreen explanation.Matthew Vaughn's attitude towards first Class of
It being part prequel to Singer's films and one part reboot meant they would be continuity errors.I have little doudt that If Singer would have been able to direct First Class It
would fit better with other films,or at least to X-men and X2.I can still view first Class,X-men,and X2 pretty well together despite the errors.

Whats funny is. All they have to do is remove any dates that pop up on the screen & maybe remove some dialogue here & there. Then everything is up in the air with how everything connects